linux-kernel - Re: [RFC PATCH 19/21] KVM: gmem: Split huge boundary leafs for punch hole of private memory

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Message-ID: <5749b89695679d21542efea03035dc5a682fbbeb.camel@intel.com>
Date: Tue, 13 May 2025 22:59:01 +0000
From: "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>
To: "pbonzini@...hat.com" <pbonzini@...hat.com>, "seanjc@...gle.com"
	<seanjc@...gle.com>, "Zhao, Yan Y" <yan.y.zhao@...el.com>
CC: "Shutemov, Kirill" <kirill.shutemov@...el.com>, "quic_eberman@...cinc.com"
	<quic_eberman@...cinc.com>, "Li, Xiaoyao" <xiaoyao.li@...el.com>,
	"kvm@...r.kernel.org" <kvm@...r.kernel.org>, "Hansen, Dave"
	<dave.hansen@...el.com>, "david@...hat.com" <david@...hat.com>,
	"thomas.lendacky@....com" <thomas.lendacky@....com>, "tabba@...gle.com"
	<tabba@...gle.com>, "Li, Zhiquan1" <zhiquan1.li@...el.com>, "Du, Fan"
	<fan.du@...el.com>, "linux-kernel@...r.kernel.org"
	<linux-kernel@...r.kernel.org>, "michael.roth@....com"
	<michael.roth@....com>, "Weiny, Ira" <ira.weiny@...el.com>, "vbabka@...e.cz"
	<vbabka@...e.cz>, "binbin.wu@...ux.intel.com" <binbin.wu@...ux.intel.com>,
	"ackerleytng@...gle.com" <ackerleytng@...gle.com>, "Yamahata, Isaku"
	<isaku.yamahata@...el.com>, "Peng, Chao P" <chao.p.peng@...el.com>,
	"Annapurve, Vishal" <vannapurve@...gle.com>, "jroedel@...e.de"
	<jroedel@...e.de>, "Miao, Jun" <jun.miao@...el.com>, "pgonda@...gle.com"
	<pgonda@...gle.com>, "x86@...nel.org" <x86@...nel.org>
Subject: Re: [RFC PATCH 19/21] KVM: gmem: Split huge boundary leafs for punch
 hole of private memory

On Thu, 2025-04-24 at 11:08 +0800, Yan Zhao wrote:
> +static int kvm_gmem_invalidate_begin(struct kvm_gmem *gmem, pgoff_t start,
> +				     pgoff_t end, bool need_split)
>  {
>  	bool flush = false, found_memslot = false;
>  	struct kvm_memory_slot *slot;
>  	struct kvm *kvm = gmem->kvm;
>  	unsigned long index;
> +	int ret = 0;
>  
>  	xa_for_each_range(&gmem->bindings, index, slot, start, end - 1) {
>  		pgoff_t pgoff = slot->gmem.pgoff;
> @@ -319,14 +320,23 @@ static void kvm_gmem_invalidate_begin(struct kvm_gmem *gmem, pgoff_t start,
>  			kvm_mmu_invalidate_begin(kvm);
>  		}
>  
> +		if (need_split) {
> +			ret = kvm_split_boundary_leafs(kvm, &gfn_range);

What is the effect for other guestmemfd users? SEV doesn't need this, right? Oh
I see, down in tdp_mmu_split_boundary_leafs() it bails on non-mirror roots. I
don't like the naming then. It sounds deterministic, but it's really only
necessary splits for certain VM types.

I guess it all depends on how well teaching kvm_mmu_unmap_gfn_range() to fail
goes. But otherwise, we should call it like kvm_prepare_zap_range() or
something. And have it make it clearly do nothing for non-TDX high up where it's
easy to see.

> +			if (ret < 0)
> +				goto out;
> +
> +			flush |= ret;
> +		}
>  		flush |= kvm_mmu_unmap_gfn_range(kvm, &gfn_range);
>  	}
>  
> +out:
>  	if (flush)
>  		kvm_flush_remote_tlbs(kvm);
>  
>  	if (found_memslot)
>  		KVM_MMU_UNLOCK(kvm);
> +